1. Field of the Invention
This invention relates to an improvement in solid propellant rocket motors, and more particularly, to such motors that are encased in a composite or filament-wound material.
2. Description of the Prior Art
Solid propellant rocket motors are known in the prior art in which the casing for the propellant is fabricated from strong filaments in a matrix of a curable polymer. A casing made in this way is known in the trade as a composite or filament-wound casing. Because of the nature of the composite material, water vapor from the atmosphere, over a long period of time, can permeate the casing. Permeation of the casing with water vapor causes the propellant bond to the case and/or intermediate liner between the case and propellant to fail. Such failure results in an unusable rocket motor. If used, the rocket motor can fail catastrophically.
A metal foil is the most effective known barrier against such water vapor or moisture permeation. The application of a metal coating of sufficient thickness (about 0.003 inches or 0.008 centimeters, minimum) to a rocket motor composite casing is difficult, however, and presents a number of problems. First, for reasons of safety, ion-vapor deposition of a metal to the composite casing followed by a plating process cannot be effected after the rocket motor casing has been loaded with propellant. Nor can such metallic coating processes be performed on the motor casing before loading of the propellant. This is because of degradation of the casing that could result from the processing after the casing has been certified after pressure testing. The coating cannot be applied before certification with pressure testing because the pressure testing would result in expansion of the casing sufficiently to tear and debond the metallic coating, thus rendering it useless as a barrier against moisture permeation.
A preformed metallic coating adhesively applied to the surface of a rocket motor composite casing after loading could provide an effective barrier to water vapor permeation through the casing. Attempts to use such preformed coatings, however, have also been beset with problems, particularly in respect of the application of such preformed coatings to the ends of the casing which normally comprise a hemispherical dome of generally spherical shape.
For the cylindrical surfaces of the casing, or other surface areas of regular shape, a preformed metal coating can consist of a metal foil adhesively-backed tape such as aluminum tape. The cylindrical and regular surfaces can be covered with the tape spirally wrapped around the case and overlapped sufficiently to effectively prevent the passage of water vapor.
The hemispherical domes on the ends of the casing, however, cannot be covered with foil tape. This is because the foil tape cannot be applied without wrinkles. Wrinkles can cause cracks in the tape that allow moisture vapor passage thus rendering the foil tape useless as a barrier against water vapor permeation.
Another factor requiring consideration in the use of a metallic moisture barrier is low weight. Any weight not absolutely required to cause the rocket motor to operate properly diminishes the efficiency thereof.
It has been proposed in the prior art to fabricate free standing metal shells for adhesive bonding to the dome ends of a composite cased solid propellant rocket motor. Such preformed shells are of complex shape and must have sufficient thickness to prevent the passage of moisture. They must also be of the lightest practical weight which is structurally strong enough to allow handling and adhesive bonding to the rocket motor case. Attempts made in the prior art to fabricate such dome covers or shells have not been successful. One technique that has been tried is spin forming. Spin forming is widely used to form complex metallic shapes and would be satisfactory except that the thickness of metal required to use this technique is approximately 0.060 inches, minimum. This is about ten times the thickness, and consequently, the weight that is desired for metallic shells to cover the dome ends of a composite cased solid propellant rocket motor.
Thus, there is a need and a demand for an improved method and apparatus that overcome the aforementioned difficulties that have been encountered in the prior art for preserving composite material cased solid propellant rocket motors against the deleterious effects of water vapor in the atmosphere. The present invention was devised to fill the technological gap that has existed in the art in this respect.